The immune system plays a critical role in the pathogenesis of pain triggered by injury or illness. A precise understanding of the mechanisms through which particular immune mediators contribute to sensitization of nociceptive neuronal pathways will be essential for the development of more efficacious treatment strategies. The complement system is a principal component of innate immunity. This proposal focuses on two highly active split products of the complement system, C3a and C5a. Increased production of C3a and C5a has been reported in various pathological states associated with pain, including arthritis, pancreatitis, inflammatory bowel disease, burns and surgical trauma. Blocking the synthesis of C3a and C5a or antagonizing their receptors produces analgesic effects in animal models of inflammatory, neuropathic and postoperative pain. Moreover, direct administration of the complement fragments increases nociceptive sensitivity to heat and mechanical stimuli in animal models. In spite of this strong evidence implicating the complement system in the development of pain hypersensitivity, the underlying mechanisms are not understood. The goal of this project is to investigate mechanisms that are responsible for the enhanced pain sensitivity produced by the generation of C3a and C5a in the affected tissues. We hypothesize that C3a and C5a receptors are expressed and functionally coupled with TRPV1 in a subset of primary nociceptors; activation of these receptors sensitize nociceptors via protein kinase C-dependent modulation of the TRPV1 channel, which is known to function as a molecular integrator of pain producing stimuli. We will use a multidisciplinary approach involving immunohistochemistry, single-cell RT-PCR, [Ca2+]i imaging, patch-clamp analysis, single-fiber recordings and measurement of nociceptive behavior to test our central hypothesis. In Aim 1, we will characterize the expression and subcellular distribution of C3a and C5a receptors (C3aR and C5aR, respectively) and TRPV1 in various classes of sensory neurons identified by specific molecular markers. We will also use Ca2+ imaging and patch-clamp recordings to test functional coupling of TRPV1 with C3aR and C5aR. In Aim 2, we will investigate intracellular signaling cascades that link the activation of C3aR and C5aR with TRPV1 sensitization. In Aim 3, we will use single-fiber recordings and behavioral studies to examine the role of PKC- dependent modulation of TRPV1 downstream of C3aR and C5aR activation in regulating nociceptor excitability as well as heat and chemical sensitization of nociceptors. The proposed studies will help to characterize the novel roles of C3a and C5a receptors in the regulation of nociceptor function, and may lead to the development of new therapeutic strategies targeting the complement system to alleviate pain.